Integral junk basket for drill bit

ABSTRACT

An integral junk basket attached directly to a rotary drill bit comprises a cylindrical shell surrounding and radially spaced from the bit shank to provide an annular cup open at the upper end. The cylindrical shell is attached at its lower end to the drill bit adjacent to the drilling head, preferably by circumferential fillet welds. The shell alternatively may be threaded or rolled onto the drill bit or attached by a snap ring or a resilient O-ring. The integral junk basket eliminates the need for additional subassemblies and does not reduce the strength of any member in the drill string.

[4 1 Sept. 23, 1975 United States Patent 11 1 Thompson et al.

[ INTEGRAL JUNK BASKET FOR DRILL BIT 2,801,079 2,894,725 Baker............

[75] Inventors: Charles T. Thompson, Dallas, Tex.;

Baker.....,...... Frank Kidd New Iberia, fl Jackson 175/308 Lawrence W. Diamond, Richardson, Kanady er al..... 175/308 Tex. [73] Assignee: American Coldset Corporation, Primary ExaminerJ?meS Leppink Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin Dallas, Tex.

[22] Filed: Apr. 23, 1974 [57] ABSTRACT An integral junk basket attached directly to a rotary Appl. No.: 463,278

drill bit comprises a cylindrical shell surrounding and radially spaced from the bit shank to provide an annu- [52] [1.5. CI. Int.

lar cup open at the upper end. The cylindrical shell is attached at its lower end to the drill bit adjacent to the drilling head, preferably by circumferential fillet [58] Field of Search [56] References Cited UNITED STATES PATENTS welds. The shell alternatively may be threaded or rolled onto the drill bit or attached by a snap ring or a 175/308 resilient O-ring. The integral. junk basket eliminates 66/99 the need for additional subassemblies and does not reduce the strength of any member in the drill string. Middleton et a1. 175/312 10 Claims, 10 Drawing Figures mm mm mm W H n P. a e .1

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US Patent W wa Z a; T w 1 INTEGRAL JUNK BASKET FOR DRILL BIT BACKGROUND OF THE INVENTION This invention relates to earth boring equipment and more particularly to junk baskets for use with rotary drills. t

Junk baskets have been used for many years in earth boring operations to trap broken parts of drill bits, such as drill teeth, broken cones, or roller bearings from roller type drill bits or tungsten carbide insert teeth from blade type drills. In addition, other items categorized as junk may include claptrap dropped into the hole from the surface such as tongs, pieces of chain, and nuts and bolts, as well as broken stabilizer blades and similar items that are too heavy to be carried by the drilling mud or other circulating fluid to the top of the hole. Without a junk basket to catch them, such articles tend to fall to the bottom of the hole and clog or damage the drill bit.

Junk baskets are usually in the form of an annular cup open at the top, and prior baskets typically have been made as part of a separate subassembly attached between the end of the drill pipe and the drill bit. For example, U.S. Pat. No. 2,670,930, issued Mar. 2, 1954 to W. W. Farrar, discloses a combination fishing tool and circulating jars for attachment to the lower end of a drill stem and carrying a rotary drill bit at its lower end. The fishing tool is in the form of ajunk basket positioned above the circulating jars and separated by a substantial axial discrete from the drill bit.

In another U.S. Pat. No. 2,801,079 issued to F. Gress on July 30, I957, a cylindrical cup is mounted through a central hole in its bottom wall onto a threaded pin extending from the lower end of a drill stem. A fishtail bit is screwed onto the threaded pin and clamps the cup against a downward-facing shoulder on the drill stem.

Other prior art junk basket designs for use with rotary drills include a three-piece junk basket subassembly shown in U.S. Pat. No. 2,894,725 issued July 14, 1959 to R. C. Baker and a two-piece basket and collar arrangement shown in U.S. Pat. No. 3,102,600 issued Sept. 3, 1963 to J. G. Jackson. A telescoping subassembly for selectively tranferring drill cuttings from an upper annular junk basket to a lower central cuttings basket is shown in U.S. Pat. No. 3,118,510, issued to W. E. Kanady et al. on Jan. 21, 1964.

All of the devices of the foregoing patents are relatively complex, with the exception of the cup arrangement shown in the Gress patent. All of these devices without exception include a junk basket as a separate subassembly mounted at varying distances above the drill bit. Because of the vertical separation between the drill face and the lip of the basket or cup in these prior designs, heavierjunk may not be carried up far enough by the circulating fluid to be trapped in the basket. This can be a particularly undesirable situation when using diamond drill bits.

The prior art junk baskets that are part of a separate subassembly also have the disadvantage of creating a weak link in the drill string. Rotary drilling apparatus. particularly the kind used for de ep'hole drilling, must transmit heavy torque forces. Typically. therefore, the drill string is made of pipe having the largest possible diameter that will still provide sufficient annular flow area between the outside of the pipe and the wall of the hole for the circulating liquid to return to the surface without excessive pressure loss. Typically the rotary drill bit has an enlarged drilling head and a shank of reduced diameter approximately equal to the diameter of the drill pipe. The shank of the bit usually is externally threaded at its upper end to mate with internal threads cut in the lower end of the drill pipe.

Any subassembly inserted between the end of the drill pipe and the drill bit should have preferably an outer diameter no larger than the outer diameter of the drill pipe and the bit shank to avoid constricting the annular flow passage for the drilling fluid. In order to provide annular space for a catch basket inside this maximum permissible diameter, therefore, it is necessary to have a necked down portion in the region of the junk basket. All the torque to the drill bit must be transmitted through this portion of reduced diameter, which is inherently the weakest point in the string.

This disadvantage is partially compensated for in the design of the Baker U.S. Pat. No. 2,894,725 described above by providing heavy radial ribs between the necked down portion and the wall of the junk basket, but the ribs necessarily obstruct the opening to and capacity of the junk basket.

SUMMARY OF THE INVENTION The integral junk basket design of the present invention overcomes the disadvantages of the prior art arrangements by attaching the basket directly to a rotary drill bit of the type having an enlarged head with cutting elements mounted thereon and a cylindrical shank of reduced diameter for attachment to the lower end of a drill pipe. The bit may have conventional roller cutters of may be of the multibladed type set with diamonds or other hard cutting elements. Both roller and diamond bits usually have a tapered shoulder section joining the enlarged head portion to the shank.

In this invention the junk basket comprises a thin cylindrical shell concentrically surrounding the shank of the drill bit, the internal surface of the shell being spaced radially from the outer surface of the shank to provide an annulus open at the upper end. The cylindrical shell is attached at its lower end to the drill bit adjacent to the tapered shoulder that joins the shank to the head. The outer diameter of the basket shell is preferably nearly equal to the diameter of the enlarged head, and the shell is preferably. attached to the bit by circumferential fillet welds that are subsequently ground smooth.

In this manner, the basket shell forms an upwardextending skirt-like continuation of the enlarged head; so that the circulating fluid that is used to lubricate and cool the cutting elements and to carry particles away from the face of the drill will flow smoothly, and with substantially higher uniform velocity, upward through the annular space from the cutting face of the bit to the lip of the basket. Thus there will be little tendency for even the heaviest particles to settle out prior to reaching the level of the basket lip.

The upward flow velocity abruptly decreases, however, as the annular passageway widens above the lip, and the flow tends to curl radially inward, thereby facilitating capture and retention in the basket of the heavier particles entrained in the circulating fluid stream. Since the annular flow area is substantially con stant for the remaining distance to the surface, those materials that do not drop into the basket will tend to be carried to the top of the hole by the upward flowing stream of fluid.

Alternative means of fastening the shell of the junk basket to the drill bit may be used if desired. For exam ple, the lower portion of the shell may be interiorly threaded and mating threads cut in the outer surface of the cutting head adjacent the shoulder section, or a circumferential groove may be formed in the outer surface of the cutting head near the shoulder and the basket shell may then be pressed by a rolling or spinning operation into mating engagement with the groove. Other possible attachment means include snap rings or resilient O-rings.

Whatever the means of attachment, the cylindrical shell is intended to form, in combination with a drill bit of the type described, an integraljunk basket to be used for the life of the drill bit and then discarded. Because the basket design of the invention makes use of the existing configuration of rotary drill bits, little or no machining is necessary to add an integral junk basket to conventional drill bits in the field. Also, because there is no significant removal of material from the drill bit to create the integral junk basket, there is no reduction in strength of any element of the drilling system. Finally, because of the simplicity of the integral design, the junk basket of the present invention is cheaper to manufacture than the more elaborate subassemblies of the prior art.

Accordingly, it is a principal object of this invention to provide ajunk basket for rotary earth-drilling equipment that is attached directly to a rotary drill bit as close as possible to the cutting head without additionally restricting the upward flow of circulating fluid from the cutting face of the bit or reducing the strength of any component of the drilling system.

A further object of the invention is to provide means for attaching a junk basket directly to a radial drill bit without making substantial structural changes to the bit. I

A still further object of the invention is to provide in combination with a rotary drill bit an integral junk basket of simple design and inexpensive construction that is usable for the life of the bit and is intended to be discarded when the bit is too worn for further use.

These and other objects, advantages, and features of the invention will become more apparent from the following description of the preferred embodiments in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a rotary drill bit in combination with an integral junk basket according to the invention.

FIG. 2 is a side view of the embodiment of FIG. 1 in cross section, taken along lines 2-2, showing the device in operative position at the bottom of a bore hole.

FIG. 3 is a top view in cross section of the embodiment of FIG, 1, taken along lines 3-3 of FIG. 2.

FIGS. 4 through are partial side views in cross section showing alternative means for attaching the junk basket shell to a rotary drill bit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. l3, a combination rotary drill bit and integral junk basket 11 according to the invention includes a drill bit 12 having an enlarged head 13 joined to a shank 14 of reduced diameter. The upper end of shank 14 has a tapered externally threaded portion 15.for engagement with mating internal threads of the lower end of a drill pipe 16. A cylindrical shell 17 extends upward from the enlarged head of the drill bit in coaxial, radially-spaced relation to shank 14, the 5 space between the inside of the shell and the outer surface of the shank forming an annular cup or basket.

Drill bit 12 is shown as a diamond-type bit of conventional design similar to that disclosed in US. Pat. No. 3,599,763 issued on Aug. 17, l97l to Charles T. Thompson. As shown in-FIG. 2, the bit includes a central passageway 18 connecting the inside of drill pipe 16 to a plurality of'grooves 19 that extend outward across the cutting face of the bit to join fluted passages 20 formed between lands 21 circumferentially spaced around the outside of enlarged head 13. Drilling mud or other fluid medium is pumped down through the drill pipe, circulates across the cutting face to cool and lubricate the cutting surfaces, and carries particles of rock upward through the annular space between the drill pipe and wall 22 of the bore hole back to the surface.

The concave face, crown, and tapered portions of the lands of the drill head are inset with diamonds 23 or other hard cutting particles, such as tungsten carbide inserts. Any junk at the bottom of the hole tends to be carried upward by'the circulating fluid as it flows with relatively high velocity through the fluted passageways 20. At the point in conventional bits where the enlarged head joins the shank of the bit, the sudden increase in cross-sectional flow area creates a corresponding decrease in flow velocity. The junk tends to settle out of the fluid stream at this point and to fall back between the lands and the bore hole where it gouges the walls of the hole and the outer surface of the lands and increases the required drilling torque.

Conventionaljunk baskets of the type described ear lier, which form part of a subassembly connected between the drill pipe and the drill bit, are located a substantial distance above the onset of this lower velocity region and thus fail to catch many of these particles. As shown in FIG. 2, however, the outer surface of basket shell 17 forms a smooth continuation of enlarged head 13; so that there is no significant drop in fluid velocity until the flow reaches upper lip 24 of the integral junk basket.

The sharp edge of this lip and the annular cavity between the shell and the shank create inward curling vortices at this point that tend to carry the heavier particles entrained in the upward flowing fluid into the junk basket. These vortices also act as a barrier to prevent any particles, carried for an additional distance upward by the fluid before they fall back, from reentering the space between'shell l7 and the wall of the bore hole. 1

The circulating vortex action at the lip of the junk basket is reinforced by providing a plurality of circumferentially spaced holes 25 drilled at a downward angle through shell 17 to permitfluid to escape from the basket back to the region of high velocity and lower pressure, as shown ,byjthe arrows.

Although the integral junk basket is shown in the figures as attached-to a diamond-type drill bit, it is equally adapted for use with conventional roller cutter bits. These bits also normally have an enlarged head in which beveled teeth-like cutters are rotatably mounted. A typical example of such a bit is shown in FIG. 1B of the Kanady et al. U.S.-Pat: No. 3,121 8,5l0 described earlier. it

The integral junk basket of this invention is mounted on such a bit just as on the-bit f FIGSQll -3 herein, namely, so that it forms an upward skirt-like extension of the enlarged head portion. Preferably, the basket shell should have an outsidediameter nearly equalto the diameter of the drill head so there will be no change in flow cross section, and therefore no change inflow velocity, from the cutting face to the basket lip In this way there will be no localized regions of low velocity below the basket where some particles might tend to get trapped.

Referring next to FIGS. 4 through 10, a number of alternative means are shown for attaching the shell of the junk basket to the drill bit. In all of the embodiments, except that of FIG. 8, shell 17 is formed simply as a short length of thin walled tubing having an outside diameter equal to the outside diameter of the drill head. A relieved band is cut into the upper shoulder of the drill head for a short axial distance to provide a piloting surface 26 for the inside diameter of the shell and a shoulder 27 for the lower end of the shell. This permits the shell to fit snugly in proper position on the bit and to present a substantially unbroken surface extension of the drill head. v.

In FIG. 4 the shell is attached to the drill by means of circumferential outside fillet weld 28 and inside fillet weld 29. Preferably, the outside weld is ground flush with the surface of the drill head to provide a smooth unbroken surface. The inner weld may be omitted.

In FIG. the shell is attached to the drill by circumferentially spaced plug welds 30, ground smooth and flush with the outer surface of the shell. This attachment means will provide fully satisfactory results at lower cost than the fillet welds.

In FIG. 6, the basket shell and piloting surface of the drill bit are provided with mating threads. This attachment means allows the shell to be removed from the bit, if desired. On the other hand, for permanent attachment, this threaded embodiment can be augmented with tack welds, plug welds, or continuous fillet welds.

In the embodiment of FIG. 7, mating grooves 31 and 32 are machined in the shell and pilot surface of the bit, respectively. An expanding snap ring 33 is fitted in groove 32 and the shell pushed down over the pilot surface until groove 31 is brought opposite to groove 32. This allows the snap ring to expand into groove 31 and to permanently lock the shell onto the bit.

FIG. 8 shows an alternative snap ring arrangement which requires a shell having a shouldered portion 34. A groove 35 is cut in the shank portion of the drill bit just above the top surface of shoulder 34 when the basket shell is fully seated on pilot surface 26 against shoulder 27 of the bit. A flat ring 36 snaps into the groove 35 to hold the basket shell in place. The ex posed shoulder snap ring of FIG. 8 permits the basket shell to be removed, if desired, while the blind snap ring of FIG. 7 creates a permanent attachment.

Still another means for permanently attaching the shell to the drill bit is shown in FIG. 9. A round groove 37 is turned in the pilot surface 26, and a shell of malleable material such as steel or aluminum is fitted in place over the pilot surface. The shell is then deformed into the groove by using conventional rolling or spinning techniques to create a simple yet effective attachment.

Finally, in FIG. 10 a variant of the blind snap ring of FIG. 7 is shown in which an. elastomeric O-ring 38 is used in placeof snap ring 33 of FIG. 7. The O-ring is compressible to allow the shell to slide over it after it has, been placed into a groove 39 cut into the pilot surface of the bit. When a mating groove 40 in the shell aligns the groove 39, the O-ring expands to hold the shell in place.-

The shell of the junk basket can be made of any suitable material such as mild steel, aluminum, plastic or fiber glass, and other conventional means of attaching the shell to the bit can be used, if desired. The junk basket can be formed as an integral part of the drill bit at the time of manufacture, but it has the additional advantage of being readily and inexpensively adaptable to retrofit installation in the field.

From the foregoing description it can be seen that the integral junk basket of the present invention provides several advantages over the designs of the prior art. It is simple and inexpensive. It requires no additional components to be inserted in the drill string between the bottom of the drill pipe and the bit. It does not reduce the strength of any element of the drill string. Finally, as a result of its location as close as possible to the drill face and of its presenting a smooth skirt-like extension of the enlarged head portion, the junk basket of this invention is considerably more effective at catchingjunk as well as particles of other materials that could damage or jarn the drill.

We claim:

1. A combination rotary drill bit and junk basket comprising:

a cylindrical shank portion threaded at one end for engagement with mating threads at the lower end of a drill pipe;

an enlarged cylindrical head portion integrally formed at the other end of the cylindrical shank portion, the head portion having a diameter larger than the diameter of the shank portion and a plurality of eircumferentiall y spaced, longitudinally extending lands formed on the head portion, with discrete hard cutting elements embedded in the end of the head portion and on the outer surfaces of the lands for drilling a bore hole larger than the diameter of said head;

a cylindrical shell portion surrounding the shank portion, the shell having an outer diameter equal to the diameter of the head portion and an inner diameter larger than and radially spaced from the shank portion;

means for attaching one end of the cylindrical shell portion to the drill bit so that the shell portion forms an integral constant diameter, smoothly faired skirtlike extension of the head portion, with the space between the shell portion and the shank portion defining an annular cup opening toward the threaded end of the shank portion; and

a passageway through the center of the drill bit opening to the cutting end of the head for conducting drilling fluid supplied from the drill pipe to the cutting cnd of the bit and thence upward between the outer surface of the enlarged head and the wall of the bore hole. the constant diameter of the head and skirt causing substantially constant flow velocity until the stream of fluid passes the skirt lip,

thereby tending to prevent junk entrained in the stream from falling back until it has passed the lip of the cup. 2. The junk basket combination of claim 1 further comprising a plurality of spaced passageways through the shell extending at acute angles with respect tothe axis of the drill bit in a direction away from the head of the bit.

3. The combination drill bit and junk basket of claim 1 wherein the means for attaching the shell portion of the drill bit comprises a circumferential fillet weld joining the one end of the shell portion to the head portion. 4. The combination drill bit and junk basket of claim 3 wherein the means for attaching the shell portion to the drill bit further comprises an additional circumferential fillet weld joining the interior surface of the shell portion to the head portion.

5. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the pilot surface having a circumferential groove therein, and the inner surface of the cylindrical shell having a mating groove; and the means for attaching the shell portion to the drill bit comprises a ring member, deformable to fit within one of said mating grooves for allowing the cylindrical shell to slidably engage the pilot surface and tending, when undeformed, to fill at least a portion of both of said mating grooves for locking the shell portion to the head portion.

6. The combination drill bit and junk basket of claim 5 wherein the deformable ring comprises a metal snap ring.

7. The combination drill bit and junk basket of claim 5 wherein the deformable ring comprises an clastomcric O-ring.

. 8. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at ,the one end thereof, the shell portion includes an internal shouldered portion located intermediate the ends thereof and having an internal diameter for slidably engaging the surface of the shank portion, the shank portion includes an exposed circumferential groove located adjacent to the shouldered portion of the shell when the inner surface of the shell portion fully engages the pilot surface, and the means for attaching the shell portion to the drill bit comprises a snap ring sized to fit in the circumferential groove and to extend radially outward therefrom for bearing against the shouldered portion of the shell.

9. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the pilot surface having a circumferential groove therein; and the means for attaching the shell portion to the drill bit comprises a circumferential region of the shell portion that is radially offset inward into mating engagment with the circumferential groove in the pilot surface of the head portion.

10. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the shell portion having a plurality of circumferentially spaced holes therethrough in the region of the pilot surface; and the means for attaching the shell portion to the drill bit comprises a plurality of plug welds filling said plurality of holes through the shell portion.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,907 047 Dated Sentember 2'4 197';

Inventor(s) Charles T. Thompson, et 211 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 30, "discrete" should read --distance Column 2, line 32, "of may be" should read or may be Column 4, line 1, "of" should read at Signed and Scaled this tenth D 3) Of February 1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Patents and Trademarks 

1. A combination rotary drill bit and junk basket comprising: a cylindrical shank portion threaded at one end for engagement with mating threads at the lower end of a drill pipe; an enlarged cylindrical head portion integrally formed at the other end of the cylindrical shank portion, the head portion having a diameter larger than the diameter of the shank portion and a plurality of circumferentially spaced, longitudinally extending lands formed on the head portion, with discrete hard cutting elements embedded in the end of the head portion and on the outer surfaces of the lands for drilling a bore hole larger than the diameter of said head; a cylindrical shell portion surrounding the shank portion, the shell having an outer diameter equal to the diameter of the head portion and an inner diameter larger than and radially spaced from the shank portion; means for attaching one end of the cylindrical shell portion to the drill bit so that the shell portion forms an integral constant diameter, smoothly faired skirt-like extension of the head portion, with the space between the shell portion and the shank portion defining an annular cup opening toward the threaded end of the shank portion; and a passageway through the center of the drill bit opening to the cutting end of the head for conducting drilling fluid supplied from the drill pipe to the cutting end of the bit and thence upward between the outer surface of the enlarged head and the wall of the bore hole, the constant diameter of the head and skirt causing substantially constant flow velocity until the stream of fluid passes the skirt lip, thereby tending to prevent junk entrained in the stream from falling back until it has passed the lip of the cup.
 2. The junk basket combination of claim 1 further comprising a plurality of spaced passageways through the shell extending at acute angles with respect to the axis of the drill bit in a direction away from the head of the bit.
 3. The combination drill bit and junk basket of claim 1 wherein the means for attaching the shell portion of the drill bit comprises a circumferential fillet weld joining the one end of the shell portion to the head portion.
 4. The combination drill bit and junk basket of claim 3 wherein the means for attaching the shell portion to the drill bit further comprises an additional circumferential fillet weld joining the interior surface of the shell portion to the head portion.
 5. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the pilot surface having a circumferential groove therein, and the inner surface of the cylindrical shell having a mating groove; and the means for attaching the shell portion to the drill bit comprises a ring member, deformable to fit within one of said mating grooves for allowing the cylindrical shell to slidably engage the pilot surface and tending, when undeformed, to fill at least a portion of both of said mating grooves for locking the shell portion to the head portion.
 6. The combination drill bit and junk basket of claim 5 wherein the deformable ring comprises a metal snap ring.
 7. The combination drill bit and junk basket of claim 5 wherein the deformable ring comprises an elastomeric O-ring.
 8. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the shell portion includes an internal shouldered portion located intermediate the ends thereof and having an internAl diameter for slidably engaging the surface of the shank portion, the shank portion includes an exposed circumferential groove located adjacent to the shouldered portion of the shell when the inner surface of the shell portion fully engages the pilot surface, and the means for attaching the shell portion to the drill bit comprises a snap ring sized to fit in the circumferential groove and to extend radially outward therefrom for bearing against the shouldered portion of the shell.
 9. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the pilot surface having a circumferential groove therein; and the means for attaching the shell portion to the drill bit comprises a circumferential region of the shell portion that is radially offset inward into mating engagment with the circumferential groove in the pilot surface of the head portion.
 10. The combination drill bit and junk basket of claim 1 wherein the enlarged head portion includes a cylindrical pilot surface concentric with the shank portion and slidably engaging the inner surface of the shell portion at the one end thereof, the shell portion having a plurality of circumferentially spaced holes therethrough in the region of the pilot surface; and the means for attaching the shell portion to the drill bit comprises a plurality of plug welds filling said plurality of holes through the shell portion. 